The photoreceptors of the retina represent the first step of vision. This project will use advanced adaptive optics imaging techniques to bypass the resolution limits imposed by the eye's optics in order to measure cellular structure and function in the living human retina. Using the techniques already developed in our lab we will be able to probe photoreceptors with unprecedented precision. In addition, we will further develop Adaptive Optics imaging techniques to improve our existing ability to image the rod and cone photoreceptors of the eye, with the goal of imaging every photoreceptor in large regions of the posterior pole in a single subject session. This will be achieved using dual wavelength imaging where we use near infrared light to image the cones at high contrast, and use simultaneously acquired low intensities visible light images to provide higher resolution images of rods and cones through image averaging. The ability to use visible light at lower intensities will also allow us to directly measure the amount of photopigment in the eye, and how the quantity of photopigment changes with bleaching and regeneration. Finally, by using the high quality images to dynamically guide a retinal tracking algorithm, we will probe local retinal sensitivity over abnormal regions of the eye using a dual image stabilization technique and probing retinal sensitivity on a purely local basis. We will use the above techniques to ask a specific series of questions. These questions focus on the control of retinal light absorption, and the effects of aging and disease on the ability of the photoreceptors to collect light. The first question addresses how photoreceptor packing interacts with the amount of photopigment in the outer segments to set the relative absorptivity of cones. If cones actively control their quantum catch, then individuals with larger cones in a given area could be expected to have lower optical density of their pigment. Similarly as the eye ages, there should be adjustments in the photopigments that reflect losses in photoreceptor numbers. In the second set of experiments we will extend this analysis to patients with early age related maculopathy and early geographic atrophy testing the nature and extent of photoreceptor changes. In a final set of experiments we will pursue preliminary data showing that cones have a common phenotypic appearance across a number of early stage diseases. We will test the hypothesis that cones that are diseased, but not yet dead, have a common functional and structural appearance. If true, it would imply that this change could be an effective biomarker of a retina at risk for vision loss and for treatment efficacy.